Electronic devices contain numerous electrical circuit components, mainly transistors assembled in integrated circuit (IC) chips. Integrated circuit chips are mechanically and electrically supported on a substrate with either the active face (the surface containing the circuit elements) or the passive face confronting the substrate. This connection between the chip and its supporting substrate is referred to as first level chip interconnection.
A common method of first level chip interconnection is wire-bonding. The chip first is adhered to its substrate with an adhesive, typically in paste or film (tape) form. Depending on specific design parameters, either the active or passive face of the chip is adhered to either the active or passive face of the substrate. The active surface of the substrate carries a pattern of electrical terminals and usually electrical circuitry; however, the circuitry may be on the opposite surface of the substrate from the terminals. For the purpose of this specification, the active surface of the substrate refers to the side carrying the electrical terminals.
The substrate used in wire bonding can be either a rigid substrate, such as a metal leadframe, ceramic substrate, laminate, or it can be a flexible substrate, such as a polyimide flexible circuit.
The assembly is subjected to heat to soften the adhesive, the chip is contacted with the adhesive using light pressure, and then the assembly is put in an oven with a curing atmosphere of dry nitrogen to cure the adhesive. Typical curing schedules for epoxies are one hour at 150.degree. C.; typical curing schedules for polyimides are 30 minutes at 150.degree. C., followed by 30 minutes at 275.degree. C.
The active terminals on the surface of the chip are then connected to the active terminals on the surface of the substrate with a fine metal wire or ribbon in an automated operation known as wire-bonding.
The surface area of the applied adhesive may be smaller than the surface area of the chip in the case where the active side of the chip is attached to the substrate. This creates a gap between the substrate and the chip. After wire-bonding, this gap is filled with encapsulant by capillary action, and the encapsulant then cured. Alternatively, where the passive side of the chip is attached to the substrate, the encapsulant is used to cover the exposed active surface of the chip and the wire bonds connecting the electrical terminals of the chip with those on the substrate. Cure temperatures and times for the encapsulant typically range from 100.degree. C. to 175.degree. C. for one/half to two hours.
The encapsulated chip and substrate form what is known as a single-chip module or assembly. Other assemblies may contain multiple chips in a discrete package. For multi-chip packages, the procedure is the same for each individual chip. These packages, either the chip scale package or the multi-chip package, are in turn supported on a larger substrate, such as a printed circuit board, and interconnected with other electrical components. The interconnection between these discrete assemblies and the larger substrate is referred to as second level interconnection.
Alternatively, a bare IC chip can be directly attached to the printed circuit board in an assembly known as chip-on-board. Currently, the most widely used method for chip-on-board assembly is the wire-bonding method just described. However, the encapsulant is applied not just to the area under the chip, but also over the whole chip so as to protect it from environmental damage.
In addition to cost of materials, one of the major concerns in the semi-conductor industry is speed of manufacture, which itself translates into cost. If a process can be done faster with no loss in performance properties of the final assembly, the advantage would be an improvement welcomed by the industry.